We are currently able to talk to a Siemens ET-200S slave,
read its inputs and write to the outputs.
So the basic hardware (OSI layer 1) is implemented and works as expected.
Software (OSI layer 2 and 7) is work in progress. Most of the implemented
features work correctly.
So the Raspberry Pi acts as a DP-Master (class 1) with the current stack.

In addition to that you need to disable Device Tree support on the Raspberry Pi.
This is due to the missing Rpi-HAT-EEPROM on our board.
You can disable Device Tree support via raspi-config or by setting the following
line in /boot/config.txt:

device_tree=

Alternatively you can keep Device Tree enabled and statically configure the AtMega88 slave in
the Raspberry Pi's Device Tree.
See the Raspberry Pi documentation for details.